1. Field of the Invention
The present invention relates to an exposure control device for a camera, and more particularly, to an exposure control device for a camera in which an aperture stop is changeable and which is capable of preventing influence of changing over the aperture stop to an exposing operation.
2. Description Related to the Prior Art
A lens-fitted photo film unit is known as a simple camera in which a taking lens and shutter mechanism are incorporated and which is pre-loaded with unexposed photo film. To lower the manufacturing cost, elements in the lens-fitted photo film unit are provided with simplified structures. For example, a single shutter blade constitutes the shutter mechanism, biased by a spring to a closed position, knocked quickly toward an open position to open and shutter the shutter device.
In the lens-fitted photo film unit, an aperture stop is fixed. An f-number of the taking lens is unchanged. It may be likely that an overexposed or underexposed frame is created under a certain condition of an exposure.
To raise the image quality by overcoming such problems, U.S. Pat. No. 5,634,163 (corresponding to JP-A 9-5817) discloses an automatic exposure control with a stop changeover plate and a photometric unit. A small-diameter opening is formed in the changeover plate. The photometric unit measures brightness of a photographic object. According to the brightness, the changeover plate is shifted to change over a diameter of the aperture stop. There is a photographic light path with reference to which the small-diameter opening is movable. When the changeover plate is in an insertion position, the small-diameter opening is set in the photographic light path. When the changeover plate is in a retracted position, the small-diameter opening is shifted out of the photographic light path. A coil spring biases the changeover plate to either one of the two end positions. A solenoid magnetically attracts the changeover plate and moves the same to change the aperture stop.
The solenoid is actuated according to a photometric value. A shutter release button is adapted to start photometry upon halfway depression. A user depresses the shutter release button halfway for photometry, waits for completion of a shift of the changeover plate, and then fully depresses the shutter release button to take an exposure. This is effective in adjusting the exposure with a simple structure.
However, the automatic exposure control has a shortcoming in that the user must keep the shutter release button halfway depressed while the changeover plate is changed over. The halfway depression should be maintained for at least 10 msec. It is inevitable for the user to continue the depression much longer than this minimum time for reliability of safe photometry. This causes a wasteful use of a battery incorporated in the lens-fitted photo film unit.
If the user puts the lens-fitted photo film unit in a bag and carries the same, it is likely that the shutter release button is accidentally depressed halfway by abutment of a certain rigid article to the shutter release button of the lens-fitted photo film unit. There occurs an unwanted operation of photometry. The solenoid is likely to be powered. Power of the battery is wastefully used, to result in failure in flash emission in the lens-fitted photo film unit of which the battery is not replaceable. Furthermore, overheating of the solenoid in an electromagnet may occur by powering for a long time, to cause spontaneous combustion. This is a serious problem in view of safety of the product.
Also, operation of keeping the shutter release button halfway depressed is complicated itself. This is because most of the users who will treat the lens-fitted photo film unit are unfamiliar to complex operation of the lens-fitted photo film unit of a combined structure.
It is also probable that the user depresses the shutter release button fully without halfway depression. The shutter blade is knocked for an exposure before completion of changing over the aperture stop. Light from the object to be photographed is blocked by the shutter blade at least partially. An extremely unexposed frame may be created with low image quality. Furthermore, no object light may be incident upon the photo film.
In view of the foregoing problems, an object of the present invention is to provide an exposure control device for a camera in which the aperture stop can be changeable reliably and also a wasteful use of the battery can be prevented.
Another object of the present invention is to provide an exposure control device for a camera capable of preventing influence of changing over the aperture stop to an exposing operation.
In order to achieve the above and other objects and advantages of this invention, an exposure control device for a camera includes a shutter blade for providing an exposure by opening/shutting a photographic light path, and a shutter drive member for driving the shutter blade. There is a shutter release button. A photometric switch is turned on in response to depression of the shutter release button. A photometric circuit is operated by turning on of the photometric switch, for measuring object brightness. An aperture stop changeover mechanism changes over an aperture stop of the photographic light path according to the photographic light path. A delay structure detects completion of actuation of the aperture stop changeover mechanism and for responsively moving the shutter drive member.
According to one preferred embodiment, the delay structure includes a delay lever, released from retention upon depression of the shutter release button, for rotating in retaining the shutter drive member in the charged position, and for releasing the shutter drive member from retention upon rotation to a predetermined position to allow the shutter drive member to move to the released position, wherein the photometric switch is turned on before the delay lever rotates to the predetermined position. A governor mechanism adjusts a rotational speed of the delay lever, to block rotation of the delay lever to the predetermined position until completion of a change of the aperture stop with the stop changeover mechanism after turning on the photometric switch.
According to another preferred embodiment, the shutter drive member opens/shuts a shutter blade upon a reach to a shutter driving position. The photometric switch is turned on upon a reach of the shutter drive member to a photometric position disposed short of the shutter driving position. The delay structure includes a governor mechanism for adjusting a moving speed of the shutter drive member between the photometric position and the shutter driving position, and for moving the shutter drive member to the shutter driving position after completion of a change of the stop changeover mechanism.
According to a further preferred embodiment, the shutter drive member opens/shuts a shutter blade upon a reach to a shutter driving position. The photometric switch is turned on upon a reach of the shutter drive member to a photometric position disposed short of the shutter driving position. The delay structure includes a retention lever for retaining the shutter drive member in the photometric position, and for releasing the shutter drive member from retention upon completion of a change of the aperture stop.
According to another preferred embodiment, the delay structure includes a stop lever movable between an engaged position and a disengaged position, the stop lever, when in the engaged position, retaining the shutter drive member in the charged position, and when in the disengaged position, allowing the shutter drive member to move to the released position. An enabling lever is driven upon depression of the shutter release button, for turning on the photometric switch, then for moving the stop lever toward the disengaged position, and for moving the stop lever to the disengaged position after completion of a change of the stop changeover mechanism.
According to still another preferred embodiment, the delay structure includes an electromagnetic retainer for retaining the shutter drive member in the charged position until completion of a change of the stop changeover mechanism after depression of the shutter release button. A control circuit is operated upon turning on of the photometric switch, for causing a delay by time required for the change of the stop changeover mechanism, and for decreasing force of retention of the electromagnetic retainer to release the shutter drive member from retention.
According to another preferred embodiment, an exposure control device for a camera includes a shutter drive member is moved forwards upon a shutter releasing operation, and is moved backwards by a spring. A transmission member is movable forwards and backwards upon movement of the shutter drive member, for moving forwards without knocking a shutter blade, and for moving backwards to knock the shutter blade. A photometric switch is turned on upon movement of the shutter drive member. A photometric circuit starts measuring object brightness upon turning on of the photometric switch. A stop changeover mechanism changes over an aperture stop according to a photometric signal from the photometric circuit before the transmission member starts knocking the shutter blade.
Furthermore, a torsion coil spring connects the shutter drive member with the transmission member, the torsion coil spring is charged by rotation of the shutter drive member, and biases and moves the transmission member.
By this construction, the aperture stop can be changeable reliably, and also a wasteful use of the battery can be prevented, because the completion of actuation of the aperture stop changeover mechanism can be detected before moving the shutter drive member responsively. Furthermore, the exposure control device is capable of preventing influence of changing over the aperture stop to an exposing operation.